Same flock, different colored yolk?

We've been noticing that our brown egg layers have a very different colored yolk than our two brown leghorns, that lay white eggs. Leghorn egg yolks are light yellow, compared to everyone else who has very deep orangish color. They all have the same feed, and all free range for 8 to 10 hours a day. Is this a breed thing, or are they not getting something in their diet that the others are?